Simulation Study On Ground Stress Change Caused By Wenchuan Earthquake In Mainland China Under Background Ground Stress Field

In-situ stress is one of the important factors that affect earthquake, the research of stress state before and after the earthquake are very meaningful and have important effect for exploring earthquake gestation and occurrence mechanism.At first, we adopt China mainland three-dimensional spherical shell finite element model of power-law creep mechanism. The observed results as constraint is used to obtain China current background stress field. Then, we simulate ground stress change caused by the 5.12 Wenchuan earthquake fault brittle failure through a given critical stress value of the seismic units. The simulation results tallies with the characteristics of stress field after the Wenchuan earthquake in previous research.We use it as the goals of the model constraints that the level and orientation of maximum principal with stress shallow measurement method and deep stress focal mechanism that is a reflection of the stress type to determine the reasonable boundary conditions. The simulation of the mainland China nowadays background stress field is characterized by:(1) China continental crust shallow level of maximum principal stress direction by the north-south orientation clockwise around the Tibetan plateau tectonic with the relative motion of India, Europe Asia block and the Pacific block, and the horizontal maximum principal stress orientation consistent within 30 km on the earth’s crust. (2) The maximum horizontal principal stress direction discontinuously change near the fault zone, and the values of the maximum horizontal principal stress are affected by the feature of the faults. (3) The horizontal maximum principal stress level are within 10 MPa-80 MPa. Tibet block and west block are the highest; southeast block is the lowest; other blocks are medium. (4) The azimuth of horizontal maximum principal stress at 2000 m depth in the vicinity of Longmenshan fault zone are SE-E in the south, and NE-E in north. (5) The differential stress gradually increase from upper crust to mid-crust in Longmenshan fault zone on the vertical profile of earth crust. The differential stress below 4000 m is bigger than the lower crust of the earth crust. In conclude, shallow differential stress of fault may cause the earthquake because of the larger value.We analyze stress field change caused by fault of brittle failure, taking 5.12 Wenchuan earthquake as an example, under the current background stress field of China mainland. We establish a local coordinate system and the maximum shear failure criterion on the fault rupture surface. We simulate earthquake brittle rupture fracture by decreasing shear modulus. After Wenchuan earthquake, the crustal stress field shows that:(1) In the middle and the northern section of Longmen mountain central fault zone, the shallow maximum principal stress value has a larger decline, and the fault zone are impacted more around the focus. Falling value decreases with increasing distance from the fault zone. The maximum horizontal principal stress calculation change quantity and stress change value from real measurement is quite superficial. (2) Stress in south section of Longmen Mountain is still higher than other areas in the fault zone, and the southern section of fault is still under high stress accumulation. (3) The overall orientation of maximum principal stress is consistent on fault zone. Horizontal maximum principal stress direction is 290° in the depth of 2000 m near the focus. Compared with the results before the Wenchuan earthquake, it is 280° which shows the horizontal maximum principal stress happened clockwise rotation after the earthquake, and rotation Angle is 10° or so. Because this model can provide a reference for stress changes in the law, and calculating absolute size and orientation of the precision of the stress field change needs to be improved, the result can only be used as a reference value of stress change affected by the model error. Model precision needs to be improved.This model will provides the basis to explain long distance earthquake triggering and migration. For research of fault friction sliding failure and crush damage fault failure mechanism and the improvement of model accuracy need further work.